This invention relates to a speed sensor device and more particularly to a speed sensor which may be incorporated with an air motor to detect a threshold speed of the output shaft of a rotary vane air motor and effect cutoff of air to the motor in the event the speed of the motor becomes excessive.
The use of governors for speed control of rotary vane air motors is well known. Governor or speed control mechanisms are utilized in combination with air motor tool devices to control air flow to the motor and thus the speed of the motor. Additionally, speed sensor devices associated with the rotary shaft of an air motor are utilized to terminate air flow to air motors in the event that excessive speeds occur due to some failure in the motor.
Patents disclosing various methods for providing overspeed control for rotary vane air motors include U.S. Pat. No. 3,767,332. That patent discloses positioning of weights on opposite sides of a rotating output shaft of an air motor. At rotational speeds below a preselected level, the weights remain positioned tightly against the shaft. When the rotational speed of the shaft exceeds a preselected level, the weights are driven radially outwardly due to centrifugal force. As the weights swing outwardly, they engage a trigger mechanism which, in turn, operates a mechanical linkage to terminate the supply of air to the air motor. U.S. Pat. No. 2,586,968 discloses a similar fly weight mechanism as does U.S. Pat. No. 3,519,372.
U.S. Pat. No. 3,749,530 discloses yet another approach utilizing the centrifugal force associated with the rotating shaft of an air motor. In U.S. Pat. No. 3,749,530, a Bellville spring is responsive to the deflective forces imparted by centrifugal force thereon to bend or deflect the spring to a position which will close air inlet passages in the event of excessive speed of the output shaft of the air motor. U.S. Pat. No. 2,973,771 and U.S. Pat. No. 1,384,113 show similar devices.
While the referenced patents disclose speed sensor and speed control devices which are adequate for their purposes, a device of simpler construction with fewer parts has been sought. Such a device would be less expensive to manufacture, easier to service and more sensitive to various speeds of a rotary output shaft.